Study of open clusters within 1.8 kpc and understanding the Galactic structure (original) (raw)
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The Astrophysical Behavior of Open Clusters along the Milky Way Galaxy
The main aim of this paper is to study the astrophysical behaviour of open clusters' properties along the Milky Way Galaxy. Near-IR JHKs (2MASS) photometry has been used for getting a homogeneous Catalog of 263 open clusters' parameters, which are randomly selected and studied by the author through the last five years; most of them were studied for the first time. The correlations between the astrophysical parameters of these clusters have been achieved by morphological way and compared with the most recent works.
A study of spatial structure of galactic open star clusters
Astronomy & Astrophysics, 2002
In order to study the relation between the core and corona in galactic star clusters, the spatial structure of 38 rich open star clusters has been studied using radial density profiles derived from the photometric data of the Digital Sky Survey. The shape of the radial density profile indicates that the corona, most probably, is the outer region around the cluster. It can exist from the very beginning of the cluster formation and dynamical evolution is not the reason for its occurrence. The study does not find any relation between cluster size and age but indicates that the clusters with galacto-centric distances >9.5 kpc have larger sizes. Further, we find that the average value of the core radius is 1.3 ± 0.7 pc and that of annular width of the corona is 5.6 ± 1.9 pc, while average values of densities of cluster members in the core and corona are 15.4 ± 9.9 star/pc 2 and 1.6 ± 0.99 star/pc 2 respectively. Average field star contaminations in the core and corona are ∼35% and 80% respectively. In spite of smaller densities in the coronal region, it contains ∼75% of the cluster members due to its larger area in comparison to the core region. This clearly demonstrates the importance of the coronal region in studies dealing with the entire stellar contents of open star clusters as well as their dynamical evolution. In contrast to the cluster cores, the structure of coronal regions differs significantly from one cluster to other.
Morphology of Galactic Open Clusters
The Astronomical Journal, 2004
We analyzed the shapes of Galactic open clusters by the star counting technique with the 2MASS star catalog database. Morphological parameters such as the ellipticity and size have been derived via stellar density distribution, weighed by clustering probability. We find that most star clusters are elongated, even for the youngest star clusters of a few million years old, which are located near to the Galactic disk. The shapes of young star clusters must reflect the conditions in the parental molecular clouds and during the cluster formation process. As an open cluster ages, stellar dynamics cause the inner part of the cluster to circularize, but the overall radius gets larger and the stellar density becomes sparser. We discuss how internal relaxation process competes with Galactic external perturbation during cluster evolution.
The Average Mass Profile of Galaxy Clusters
The Astrophysical Journal, 1997
The average mass density profile measured in the CNOC cluster survey is well described with the analytic form ρ(r) = Ar −1 (r + a ρ ) −2 , as advocated on the basis on n-body simulations by Navarro, Frenk & White. The predicted core radii are a ρ = 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω = 0.2 open CDM model, or a ρ = 0.26 for a flat Ω = 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20 ≤ a ρ ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12 − 0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster RMS line-of-sight velocity dispersion alone. This is strong support of the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark matter dominated universe.
Luminosity and mass functions of galactic open clusters
Astronomy and Astrophysics, 2005
We present wide-field deep UBVI photometry for the previously unstudied open cluster NGC 4852 down to a limiting magnitude I ∼ 24, obtained from observations taken with the Wide Field Imager camera on-board the MPG/ESO 2.2 m telescope at La Silla (ESO, Chile). These data are used to obtain the first estimate of the cluster basic parameters, to study the cluster spatial extension by means of star counts, and to derive the Luminosity (LF) and Mass Function (MF). The cluster radius turns out to be 5.0 ± 1.0 arcmin. The cluster emerges clearly from the field down to V = 20 mag. At fainter magnitudes, it is completely confused with the general Galactic disk field. The stars inside this region define a young open cluster (200 million years old) 1.1 kpc far from the Sun (m − M = 11.60, E(B − V) = 0.45). The Present Day Mass Functions (PDMF) from the V photometry is one of the most extended in mass obtained to date, and can be represented as a power-law with a slope α = 2.3 ± 0.3 and (the Salpeter MF in this notation has a slope α = 2.35), in the mass range 3.2 ≤ m m ≤ 0.6. Below this mass, the MF cannot be considered as representative of the cluster MF, as the cluster merges with the field and therefore the MF is the result of the combined effect of strong irregularities in the stellar background and interaction of the cluster with the dense Galactic field. The cluster total mass at the limiting magnitude results to be 2570 ± 210 M .
The study of open clusters has a classic feel to it since the subject predates anyone alive today. Despite the age of this topic, I show via an ADS search that its relevance and importance in astronomy has grown faster in the last few decades than astronomy in general. This is surely due to both technical reasons and the interconnection of the field of stellar evolution to many branches of astronomy. In this review, I outline what we know today about open clusters and what they have taught us about a range of topics from stellar evolution to Galactic structure to stellar disk dissipation timescales. I argue that the most important astrophysics we have learned from open clusters is stellar evolution and that its most important product has been reasonably precise stellar ages. I discuss where open cluster research is likely to go in the next few years, as well as in the era of 20m telescopes, SIM, and GAIA. Age will continue to be of wide relevance in astronomy, from cosmology to planet formation timescales, and with distance errors soon no longer a problem, improved ages will be critically important to many of the most fascinating astrophysical questions.
The Dynamical Evolution of Young Clusters and Galactic Implications
Eso Astrophysics Symposia, 2009
Star clusters are observed to form in a highly compact state and with low star-formation efficiencies. If the residual gas is expelled on a dynamical time the clusters disrupt thereby (i) feeding a hot kinematical stellar component into their host-galaxy's field population, and (ii) if the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function transforms within ten to a few dozen Myr to a mass function with a turnover near 10 5 M ⊙ , thereby possibly explaining this universal empirical feature.
On the fundamental line of galactic and extragalactic globular clusters
Astronomy and Astrophysics, 2010
Context. In a previous paper, we found that globular clusters in our Galaxy lie close to a line in the (log R e , SB e , log σ) parameter space, with a moderate degree of scatter and remarkable axi-symmetry. This implies that a purely photometric scaling law exists, that can be obtained by projecting this line onto the (log R e , SB e ) plane. These photometric quantities are readily available for large samples of clusters, as opposed to stellar velocity dispersion data. Aims. We study a sample of 129 Galactic and extragalactic clusters on this photometric plane in the V-band. We search for a linear relation between SB e and log R e and study how the scatter around the best-fit relation is influenced by both age and dynamical environment. We interpret our results in terms of testing the evolutionary versus primordial origin of the fundamental line. Methods. We perform a detailed analysis of surface brightness profiles, which allows us to present a catalogue of structural properties without relying on a given dynamical model. Results. We find a linear relation between SB e and log R e , in the form SB e = (5.25 ± 0.44) log R e + (15.58 ± 0.28), where SB e is measured in mag/arcsec 2 and R e in parsec. Both young and old clusters follow the scaling law, which has a scatter of approximately 1 mag in SB e . However, young clusters display more of a scatter and a clear trend in this with age, which old clusters do not. This trend becomes tighter if cluster age is measured in units of the cluster half-light relaxation time. Two-body relaxation therefore plays a major role, together with passive stellar population evolution, in shaping the relation between SB e , log R e , and cluster age. We argue that the log R e -SB e relation and hence the fundamental line scaling law does not have a primordial origin at cluster formation, but is rather the result of a combination of stellar evolution and collisional dynamical evolution.
Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627
Monthly Notices of the Royal Astronomical Society, 2003
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins I and Washington CMT 1 systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of [Fe/H] = −0.12 ± 0.08, which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour-magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the (V, V − I) and (T 1 , C − T 1) diagrams, we estimate E(V − I) = 0.25 ± 0.05 and V − M V = 11.80 ± 0.25 for log t = 9.15, and E(C − T 1) = 0.23 ± 0.07 and T 1 −M T 1 = 11.85 ± 0.25 for log t = 9.10, respectively, assuming solar metal content. The derived reddening value E(C − T 1) implies E(B − V) = 0.12 ± 0.07 and a distance from the Sun of 2.0 ± 0.4 kpc. Using the WEBDA data base and the available literature, we reexamined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7-0.8, 1.0-1.1, 1.6-1.7 and 2.0-2.1 Gyr, separated by relative quiescent epochs of ∼0.2-0.3 Gyr. We also estimated a radial abundance gradient of −0.08 ± 0.02, which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found.